This invention relates to a method for measuring the flow rate of a liquid, particularly of blood, by determining the frequency shift of a laser beam reflected in the liquid according to the optical Doppler effect. The invention also relates to an apparatus for carrying out the method of the invention.
Nilsson, Published PCT Application No. WO 93/03667 discloses a method of the above-mentioned type and an apparatus for measuring the flow rate of a liquid, particularly of blood, specifically by determining a frequency shift of a laser beam reflected in the liquid according to the optical Doppler effect. Thus, a point-type determination of the flow rate of the blood can take place by determining the frequency shift of the reflected laser beam. This measuring method is non-invasive but permits the measuring of the flow rate only at one point. In addition, high expenditures are required on the part of the patient and of the person performing the examination; particularly since the target beam has to be aimed precisely at a defined point for an extended period of time. By means of this method, it has not been possible, particularly in the field of ophthalmology, to measure the blood flow in the vascular system of the choroid membrane.
Furthermore, Riva, U.S. Pat. No. 4,142,796 discloses a process based on the optical Doppler effect as well as a corresponding apparatus for diagnostic purposes in ophthalmology.
In many fields of medical diagnostics and therapy, it is necessary to measure blood flow rates. In particular, a considerable clinical need exists in ophthalmology to carry out a locally resolved and continuous measurement of the blood flow in the retina. It is known that the three cell layers of the retina are supplied with oxygen by two independent vascular systems. The photoreceptors of the bottom layer are supplied by the choroid membrane; the bipolar cells/amacrine cells as well as the nerve cells of the top layer are supplied by the intraretinal vascular bed. It is necessary to measure the flow rates in the afferent arterioles, the retinal capillaries and in the arteriovenous shunt vessels which divert the blood flow directly into the venules while bypassing the capillaries.
In addition to the initially mentioned Doppler laser flow rate measurement, two other methods are used clinically for determining retinal blood circulation. Thus, the very frequently carried-out fluorescence angiography is based on a qualitative assessment of the retinal arterioles and capillaries after intravenous injection of a fluorescent dye. The quantitative analysis of digitized fluorescent images supplies information on special filling times, like the arm-to-retina time or the arteriovenous passage time. However, the intravenous injection of a fluorescent dye represents an invasive procedure which entails a residual risk of an anaphylactic shock and is therefore carried out predominantly in eye clinics. Fluorescence angiography is a two-dimensionally spatially resolving procedure, but it is not resolving with respect to time and is invasive with a vital residual risk.
Finally, by means of non-invasive ultrasonic Doppler sonography (duplex sonography, pulsed Doppler sonography) the blood flow rate can be determined at limited points and resolved with respect to time in orbital arteries, arterioles and venules of a diameter of up to 1 mm. In contrast, smaller vessels and capillaries of the retina cannot be detected by means of this method.
Zinser, U.S. Pat. No. 5,170,276, the disclosure of which is incorporated herein by reference, discloses an apparatus for scanning an object by means of a bundle of rays in two essentially orthogonal directions. This apparatus contains a first and a second scanner each having a mirror, whose axes of rotation extend in planes which are orthogonal with respect to one another. This apparatus has a compact construction and contains no additional optical system in the path of the rays between the above-mentioned mirrors. The mirror of the first scanner is mounted at a defined distance from its axis of rotation. An optically perfect scanning operation is ensured primarily because the center point of the mirror of the second scanner is arranged in the center of the above-mentioned distance between the axis of rotation and the mirror of the first scanner, whereby the bundle of rays passes from the mirror of the first scanner directly to the mirror of the second scanner.
Furthermore, Chehroudi et al., "A rapidly scanning laser Doppler anemometer," J. Phys. E: Sci. Instrum., Vol. 17, pp 131-36 (1984) discloses a wind velocity indicator in which, while utilizing the Doppler effect and scanning by means of a laser beam, the wind velocity can be measured, for example, in a wind tunnel. Also, Durst et al., "Laser-Doppler system for rapid scanning of flow fields," Rev. Sci. Instrum., Vol. 52 (11), pp 1676-81 (1981) describes a Doppler laser system by means of which primarily the wind velocity also can be measured. In apparatus or systems of this type, special rigidly arranged mirrors are provided, and such systems have, as a whole, a comparatively large space requirement. Therefore, it is not readily possible to use them in medical diagnostics and therapy.